How do you transform a mesh with rig into another?
I would create one armature and one mesh, and use shape keys to turn this mesh from one shape to the other. Of course, the Dr. Banner version of the mesh may need to be rather dense, so that when it grows into the Hulk, it still looks detailed and smooth. For that reason, you may find that your system doesn't perform well when animating, and so you might need a low-res Banner mesh AND a high res Banner mesh for turning into the Hulk, with the high res version keyframed invisible/unrenderable, and all the modifiers for it deactivated, until it is needed. Both versions of the mesh would be bound to the same rig. I don't recommend trying to create two rigs. If you need the rig itself to get larger also, as I imagine in this case you would, I would keyframe the scale of the armature as a whole in Object Mode to coincide with the shape key animation change. I don't believe this will break anything, but I've never done it before. So if anyone knows of any pitfalls to this, please weigh in.